The human navel should be designated as a bacterial nature reserve, it seems. The first round of DNA results from the Belly Button Biodiversity project are in, and the 95 samples that have so far been analyzed have turned up more than 1,400 bacterial strains. In 662 cases, the microbes could not even be classified to a family, “which strongly suggests that they are new to science,” says team leader Jiri Hulcr of North Carolina State University in Raleigh.

The project was conceived as a lighthearted exercise in science communication, but it is making a serious contribution to the understanding of microbial diversity. Since April, samples of bacteria taken when volunteers swabbed their navels have had their “DNA barcodes” read by sequencing the gene for 16S ribosomal RNA, which is widely used in studies of bacterial evolutionary relationships.

The results reflect our ignorance of microbial diversity, Hulcr suggests: The inhabitants of our navels seem weird because biologists haven’t sampled extensively enough to document the full diversity of microbial life in a variety of habitats. He likens the reactions to the first round of belly button results to the astonishment of the first European explorers seeing African big game, which today seem commonplace. “Now you’re expecting rhino and elephants,” Hulcr says.

Also, identifying the species of bacteria is difficult. Noah Fierer’s team at the University of Colorado at Boulder classified them into “operational taxonomic units” having 16S ribosomal RNA gene sequences that differed by 3 percent or less. Apply this standard to mammals, Hulcr explains, and dogs and cats would be lumped together. It means that a “match” between a belly button strain and a species known from the deep ocean, for instance, may actually represent two microbes separated by several million years of divergent evolution.

Although the total number of strains recorded was large, the results so far indicate that a small group of about 40 species accounts for around 80 percent of the bacterial populations of our belly buttons. “It is tempting to think of the abundant species as the good, core biome of bacteria and the rare ones as transients, struggling to take hold, sometimes at our expense,” says Rob Dunn, author of “The Wild Life of Our Bodies” and head of the lab where Hulcr works.

Confirming that theory will require studies on a new scientific frontier: belly button ecology.

—Peter Aldhous

A longer version of this story appeared in New Scientist magazine and can be found at www.newscientist.com.

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